1. Field of the Invention
The present invention relates to a structure and manufacturing method of a chip type resistor having a resistive film and terminal electrodes positioned at opposing ends of the resistive film, formed on a chip type insulating substrate.
2. Description of the Background Art
In a conventional chip type resistor such as described in Japanese Patent Laying-Open No. 60-27104, an upper surface of a cover coat covering a resistive film formed on an upper surface of a chip type insulating substrate is protruded from a surface of terminal electrodes positioned at opposing ends of the resistive film, so that there is a considerable step between the upper surface of the cover coat and the upper surface of the terminal electrodes. Therefore, it often suffers from the problem that when the chip type resistor is soldered on a printed board with the side of the resistive film facing the printed board, one side of the chip type resistor rises or floats, preventing secure soldering of terminal electrodes at the opposing ends.
In view of the foregoing, Japanese Patent Laying-Open No. 4-102302 discloses a structure of a chip type resistor in which terminal electrodes 3 are formed at opposing ends of a resistive film 2 on left and right end portions of a chip type insulating substrate 1 such that the terminal electrodes 3 each includes a main upper electrode 3a formed on the surface of insulating substrate 1 and conductive to resistive film 2, an auxiliary upper electrode 3b formed heaped up on the surface of main upper electrode 3a, a side electrode 3c formed on either side of insulating substrate 1, and a metal plate layer 3d formed over the surfaces of auxiliary upper electrode 3b and side electrode 3c, as shown in FIGS. 28 and 29. This laid-open patent application proposes, by this structure, to reduce or eliminate the step between the upper surface of terminal electrode 3 and the upper surface of cover coat 4 using auxiliary upper electrode 3b.
Cover coat 4 has a three-layered structure including an undercoat 4a directly covering resistive film 2, a middle coat 4b covering undercoat 4a, and an overcoat covering middle coat 4b, or a two-layered structure with under coat 4a or middle coat 4b omitted.
The chip type resistor disclosed in Japanese Patent Laying-Open No. 4-102302 is manufactured through the following steps.
Step 1. On an upper surface of the insulating substrate 1, main upper electrodes 3a are formed and thereafter resistive film 2 is formed. Alternatively, resistive film 2 is formed first and thereafter main upper electrodes 3a are formed.
Step 2. Undercoat 4a of glass is formed on resistive film 2. Thereafter, resistive film 2 and undercoat 4a are engraved to form a trimming groove by laser beam irradiation, for example, while resistance value of resistive film 2 is measured by a conductive probe which is brought into contact with main upper electrodes 3a so that the resistance value of resistive film 2 is within a prescribed tolerable range.
Step 3. Middle coat 4b of glass is formed on the surface of undercoat 4a to fill the trimming groove. Thereafter, overcoat 4c of glass or a synthetic resin is formed covering resistive film 2, part of the main upper electrodes 3a, undercoat 4a and middle coat 4b.
Step 4. On the upper surface of main upper electrodes 3a, auxiliary upper electrodes 3b are formed. Thereafter, side electrodes 3c are formed on end surfaces of insulating substrate 1, and surfaces of auxiliary upper electrodes 3b and side electrodes 3c are subjected to metal plating, whereby a metal plate layer 3d is formed.
In the above described chip type resistor, auxiliary electrodes 3b are formed on the upper surfaces of the main upper electrodes 3a by directly applying the conductive paste by screen printing. Therefore, in order to reduce or eliminate the step between upper surfaces of terminal electrodes 3 and cover coat 4, auxiliary electrodes 3b must be made thick.
In order to increase thickness of auxiliary upper electrode 3b, the number of application of the conductive paste for the auxiliary upper electrodes 3b must be increased. Therefore, amount of conductive paste used is increased, resulting in considerable increase in manufacturing cost of the chip type resistor.